A cross sectional view of a conventional light emitting diode is illustrated in FIG. 1. The light emitting diode 100 includes a semiconductor substrate 102, a first ohmic contact electrode 101 formed on the rear side of the semiconductor substrate 102, a light generating region 103 formed on the semiconductor substrate 102, and a second ohmic contact electrode 106 formed on the light generating region 103. The light generating region 103 is formed by a P-type region and an N-type region, and grown on the gallium arsenide (GaAs) substrate 102. Because of the current crowding effect, limited emitting angle of the light and light absorption of the substrate, the illumination in this light emitting diode is not strong.
The crystal lattice constants in most of the light generating region 103 are matched with that of the gallium arsenide substrate. Namely, the light emitting diode of visible light is directly fabricated on the gallium arsenide substrate 102. However, since the energy gap of gallium arsenide is 1.43 eV which is smaller than that of the visible light and the light emitted from the diode is isotropic, part of the light enters the substrate and is absorbed by the gallium arsenide substrate.
U.S. Pat. Nos. 5,008,718 and 5,233,204 disclose a transparent window layer structure for increasing the output light of a light emitting diode. Referring to FIG. 2, the structure of the light emitting diode 200 is formed by a transparent window layer 204 grown on the light emitting diode 100 shown in FIG. 1. By means of the transparent window layer, the current crowding effect in a conventional light emitting diode is reduced and the current spread to emit light is increased. As a result, the illumination of the light emitting diode is enhanced.
The proper material for the transparent window layer 204 includes GaP, GaAsP, and AlGaAs, etc., whose energy gap is larger than those of the materials in the AlGaInP light generating region. Under this condition, the critical angle of the emitted light can be increased and the current crowding effect is reduced so as to enhance the illumination of the light emitting diode. However, in the electric property, since the materials on the uppermost layer of the transparent window layer 204 and the AlGaInP light generating region have a hetero junction, the energy gap difference causes the positive forward bias voltage V.sub.f of the light emitting diode to increase. As a result, the power consumption of the light emitting diode is increased.
U.S. Pat. Nos. 5.237,581 and 4,570,172 disclose a light emitting diode 300 with a multilayer reflecting structure, as shown in FIG. 3. The structure of FIG. 3 includes a semiconductor substrate 302, a lower multilayer reflector 305 formed on the semiconductor substrate 302, a light generating region 303 formed on the lower multilayer reflector 305, an upper multilayer reflector 304 formed on the light generating region 303, a first ohmic contact electrode 306 on the upper multilayer reflector 304, and a second ohmic contact electrode 301 formed on the rear side of the semiconductor substrate 302. By means of the semiconductor multilayer reflector, namely, a distributed Bragg reflector (DBR), the light transmitting to the substrate is reflected backwards so as to be emitted out of the light emitting diode. Accordingly, the light illumination is increased.
In this prior art light emitting diode, the lower multilayer reflector 305 serves to reflect 90% of the light emitted from the light generating region to the light absorption substrate, while the upper multilayer reflector serves to guide light to the upper surface of the light emitting diode. Therefore, the problem of light absorption by the substrate is alleviated, and the problem related to limited critical angle is also improved. However, since the multilayer reflector has many hetero junctions, the effect of energy gap difference is enlarged and hence the forward bias V.sub.f is increased. As a consequence, although the DBR structure disclosed in U.S. Pat. Nos. 5,237,581 and 4,570,172 can reflect the light impinging on the substrate, the DBR structure has a high reflectivity only for normal incident light (shown in D1 of FIG. 3). For oblique incident light (such as D2, D3, and D4 shown in FIG. 3) the reflectivity is decreased. Thus the improvement to the illumination of a light emitting diode in visible light band is limited, whereas the DBR structure increases difficulty in growing the thin film epitaxial layer and thus the fabrication cost.
U.S. Pat. No. 5,376,580 discloses a light emitting diode with wafer bonding, wherein a gallium arsenide substrate serves as a temporary substrate to grow a light emitting diode structure including a confinement layer, an active layer and another confinement layer. Then the light emitting diode structure is adhered to a transparent substrate, and the GaAs substrate is removed. Therefore, the problem of light absorption by a substrate can be solved completely. Because the transparent substrate disclosed in the prior art is made of GaP, a thick GaP window layer is needed to handle thin epitaxial layers properly. Consequently, the LED device is relatively thin after the GaAs substrate is removed because of the thick window layer. The light emitting diode becomes easier to break and more difficult to fabricate. Furthermore, the wafer bonding needs to be processed in high temperature for a long period of time (about 600.about.700.degree. C. for at least one hour), which results in degraded p-n junction and inferior diode performance.